1. Field of the Invention
The present invention relates to a photoelectric type displacement detection apparatus which can particularly effect detection in a stable manner with improved frequency response.
2. Prior Art
There are currently used various photoelectric type displacement detecting systems having excellent measurement accuracy and durability, such as digital display dial gauge micrometers, three-dimensional length measuring systems, table feed systems of laser mikes or NC machine tools for measuring distance or other dimensions utilizing laser beams, etc.
FIG. 1 shows the primary part of such a type of displacement detecting system, in which a main scale 10 a glass plate, a stainless steel sheet or other includes a plurality of length measuring slits 10a and absolute value indicating slits 10b formed therein and spaced away from one another. The desired length measurement or position detection may be carried out along the main scale 10.
Near the main scale 10 are disposed an index scale 12 and an absolute value scale 13. The index scale 12 includes index slits 12a and 12b corresponding to the length measuring slits 10a while the absolute scale 13 includes index slits 13a corresponding to the absolute value indicating slits 10b. Upon relative movement between the scales 10, 12 and 13 (optical lattice means), the movement of the slits is electrically detected to provide a relative movement signal on which a measurement of length or detection of position can be accomplished.
To convert the relative movement of the scales 10, 12 and 13 into a photoelectrical signal, light emitting and receiving elements ae arranged near the respective scales 10, 12 and 13. The illustrated system is a transmission type measuring apparatus in which light emitting elements 14a, 14b and 14c are located near the main scale 10, light receiving elements 16a and 16b are disposed near the index scale 12, and a light receiving element 16c is arranged near the absolute value scale 13. These elements 14 and 16 are positioned at locations opposed to the index slits 12a, 12b and 13a, respectively. As is well known, the groups of index slits 12a and 12b are offset relative to each other by a half-pitch so that light beams transmitted to each of the scales 10 and 12 will be detected as sine or cosine wave signals. These out-of-phase signals can be used to realize the desired dividing action.
The absolute value scale 13 may generate an absolute value output signal indicative of the absolute position of the moved main scale 10 in co-operation with the absolute value slits 10b provided on the main scale 10 and spaced away from one another. Thus, an operator can know an absolute position of the absolute scale 13 relative to the main scale 10.
In such a manner, the prior art system is operated to make photoelectric conversion of the displacement with the resulting relative movement signal undergoing the necessary process through such a circuit as shown in FIG. 2.
FIG. 2 shows an example of the displacement detecting system applied to a length measuring device. The displacement signal of relative movement from each of the light receiving elements 16 is amplified through a corresponding preamplifier 18a, 18b or 18c and then supplied to a detection circuit 20. The detection circuit 20 comprises a waveform shaper circuit 22 and a division circuit 24. As is well known, the division circuit 24 may combine the sine and cosine wave signals with each other to divide the combined signal into signals having a measurement pitch smaller than the slit widths of the scales 10 and 12.
The output of the detection circuit 20 is supplied to a digital display 28 through a counter 26 such that the measured length will digitally be indicated. This measured length may be recorded by a printer 30, if necessary.
The three light emitting elements 14a, 14b and 14c are connected with a power supply through resistances 31a, 31b and 31c, respectively.
In the displacement detection system mentioned above, the quantity of light from the light emitting elements 14 must be constant at all times because of its increased influence on the characteristics of the photoelectric conversion section. For example, in such a system as shown in FIG. 2, the resistances 31 connected with the respective light emitting elements 14 were finely adjusted to equalize the quantity of light from the light emitting elements 14. At the same time, if necessary, the pre-amplifier 18 could be adjusted to effect the initial adjustment of the detection output signals supplied to the detection circuit 20.
In the prior art systems, however, if the voltage at the power supply is varied, the quantity of light will correspondingly be changed in spite of the initial adjustment prior to the aforementioned measurements, resulting in frequent generation of errors in the detected values.
The prior art displacement detection systems were generally utilized to control the feed of a table in a conventional NC machine tool in which a sensor was provided in each of the NC moving shafts, these sensors being collectively controlled through a power supply, operation switch or indicator which was mounted on the stationary part of the machine.
FIG. 3 illustrates an example of such NC control systems. Each X-, Y- and Z-section of the NC machine tool includes such a photoelectric type displacement detection device as shown in FIGS. 1 and 2. Each of the axially moving parts includes a light emitting element 14a, 14b or 14c, a light receiving element 16a, 16b or 16c, and a pre-amplifier 18a, 18b or 18c. These components are housed within a movable housing 32.
On the other hand, a stationary housing 34 includes a power supply 36, a detection circuit 20 and displays 38, 40 and 42 for the respective shafts.
The movable housing 32 is provided on each of the X-, Y- and Z-sections and mounted on the corresponding NC feed shaft. The movable housing 32 is electrically connected with the stationary housing 34 through a cable 44. Such a split type detection system can include a common power supply, a common group of operation switches and a common display, resulting in reduction of dimensions in the system.
In the split type detection system, however, there are cables 44 of different lengths connecting the stationary and movable housings 34 and 32. Moreover, if a plurality of movable X-, Y- and Z-sections as shown in FIG. 3 are provided the length of the cable for each shaft will be varied during operation. Even if the voltage on the stationary side is maintained constant, the quantity of light in the light emitting elements of the respective movable sections becomes irregular when the voltage in the corresponding cable 44 lowers. As a result, errors in measurement will not be negligible.
To overcome such problems, it has been proposed to have a voltage detector and a separate power voltage control or constant power supply, all of which are housed within a movable slider unit. However, this requires a plurality of similar cables for conducting detected voltages and other complicated structures including the voltage detector, resulting in noises generated in the voltage detection and control lines. Furthermore, the separate power supply will adversely affect the movement of the slider unit and undesirably increase the dimensions of the system.
In the aforementioned construction including a plurality of movable detection sections, for example, if the voltage in the power supply on the stationary side is five volts, the actual voltage in the movable slider unit would be decreased 4%-6% due to loss in the cables resulting in 10%-15% reduction in the light quantity of the light emitting elements. In addition, the quantity of light may be reduced about 20% when it is considered that the variations in the voltage of the stationary power supply is in the range of about 5%. This provides a remarkable degradation of SN ratio in the light measuring signals and then adversely affects the accuracy in measurement.
Although the prior art systems can utilize, for the desired purpose, electric output signals indicative of information of length or position which are obtained by the photoelectric conversion of the relative movement between the main scale 10 and the index scale 12, this may raise another severe problem with respect to responsibility in recently developed high-precision detecting systems.
Moreover, reduction in power consumption is increasingly being desired, particularly, in a portable system such as battery-operated length meters and others. The photoelectric conversion systems normally provide the increase of power consumption in the light emitting elements which is desired to decrease.
In the arrangement of scales as shown in FIG. 1, it is required that the clearance between each adjacent scales moved relative to each other is increased to facilitate the assembling and machining of the components.
Due various factors mentioned above, detection systems tend to decrease the quantity of light received by the light receiving elements. For positive and sufficient phase detection even by the use of very small received signals, various ideas have been adopted in the detection circuit 20 connected with the light receiving elements.
To this end, as shown in FIG. 4, a load resistor 46 having a relatively large resistance R.sub.L is normally connected with the emitter of a light receiving element 16. A voltage V.sub.o is brought from between this emitter and the load resistor 46. This is a detected voltage having such a value that can easily be processed in the proceeding step.
However, the load resistor 46 having its large resistance R.sub.L raises a problem in that its response frequency is severely limited if the light receiving element 16 used is a phototransistor. As shown in FIG. 4, the response frequency f of the phototransistor 16 is determined by the following relationship: EQU f=1/(2.pi.Cj R.sub.L)
where Cj is the electric capacity of the phototransistor 16. As will be apparent from the above formula, increase of the load resistance R.sub.L results in reduction of response frequency f. In prior art systems, the reduction of power consumption is not compatible with improvement of response frequency.